The U.S. Food and Drug Administration on Thursday said it has cracked down on thousands of online pharmacies for selling potentially unsafe, unapproved or fake pharmaceuticals, including the erectile dysfunction drug Viagra and antiviral Tamiflu.

The FDA, working with international regulatory and law enforcement agencies from about 100 countries, said it took action against more than 4,100 Internet pharmacies, bring civil and criminal charges, removing offending websites and seizing drugs worldwide.

The move was part of the fifth annual International Internet Week of Action, a global effort to fight the online sale and distribution of potentially counterfeit and illegal medicine.

Action taken between September 25 and October 2 resulted in the shutdown of more than 18,000 illegal pharmacy websites and the seizure of $10.5 million worth of drugs.

The goal of this annual effort, which involved law enforcement, customs and regulatory authorities from 100 countries, was to identify producers and distributors of illegal pharmaceutical products and medical devices and remove these products from the supply chain.

The FDA targeted websites selling unapproved and potentially dangerous medicines. In addition to Tamiflu and Viagra, the agency targeted Domperidone, which was removed from the U.S. market in 1998 because it may cause serious adverse side effects, and Isotretinoin, previously marketed as Accutane in the United States, used to treat severe acne and carries risks, such as birth defects.